The Integumentary System Skin (Integument) Consists of two major regions 1. Epidermis superficial region 2. Dermis middle region Hypodermis (superficial fascia) deepest region Subcutaneous layer deep to skin Not one of the layers of the skin Mostly adipose tissue Epidermis Layers of the Epidermis: Stratum Basale (Basal Layer) Deepest epidermal layer firmly attached to the dermis Single row of stem cells Also called stratum germinativum: cells undergo rapid division Journey from basal layer to surface Takes 25 45 days Layers of the Epidermis: Stratum Spinosum (Prickly Layer) Cells contain a weblike system of intermediate prekeratin filaments attached to desmosomes Abundant melanin granules and dendritic cells Layers of the Epidermis: Stratum Granulosum (Granular Layer) Thin; three to five cell layers in which the cells flatten Keratohyaline and lamellated granules accumulate Layers of the Epidermis: Stratum Lucidum (Clear Layer) In thick skin Thin, transparent band superficial to the stratum granulosum A few rows of flat, dead keratinocytes Layers of the Epidermis: Stratum Corneum (Horny Layer)
20 30 rows of dead, flat, keratinized membranous sacs Three-quarters of the epidermal thickness Functions Protects from abrasion and penetration Waterproofs Barrier against biological, chemical, and physical assaults Dermis Strong, flexible connective tissue Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells Two layers: Papillary Reticular Layers of the Dermis: Papillary Layer Papillary layer Areolar connective tissue with collagen and elastic fibers and blood vessels Dermal papillae contain: Capillary loops Meissner s corpuscles Free nerve endings Layers of the Dermis: Reticular Layer Reticular layer ~80% of the thickness of dermis Collagen fibers provide strength and resiliency Elastic fibers provide stretch-recoil properties Skin Markings: Friction Ridges Epidermal ridges lie atop deeper dermal papillary ridges to form friction ridges of fingerprints Skin Markings: Cleavage Lines Collagen fibers arranged in bundles form cleavage (tension) lines Incisions made parallel to cleavage lines heal more readily Skin Color Three pigments contribute to skin color: 1. Melanin
Yellow to reddish-brown to black, responsible for dark skin colors Produced in melanocytes; migrates to keratinocytes where it forms pigment shields for nuclei Freckles and pigmented moles Local accumulations of melanin Skin Color 2. Carotene Yellow to orange, most obvious in the palms and soles 3. Hemoglobin Responsible for the pinkish hue of skin Appendages of the Skin Derivatives of the epidermis Sweat glands Oil glands Hairs and hair follicles Nails Sweat Glands Two main types of sweat (sudoriferous) glands 1. Eccrine (merocrine) sweat glands abundant on palms, soles, and forehead Sweat: 99% water, NaCl, vitamin C, antibodies, dermcidin, metabolic wastes Ducts connect to pores Function in thermoregulation Sweat Glands 2. Apocrine sweat glands confined to axillary and anogenital areas Sebum: sweat + fatty substances and proteins Ducts connect to hair follicles Functional from puberty onward (as sexual scent glands?) Specialized apocrine glands Ceruminous glands in external ear canal; secrete cerumen Mammary glands Sebaceous (Oil) Glands Widely distributed
Most develop from hair follicles Become active at puberty Sebum Oily holocrine secretion Bactericidal Softens hair and skin Hair Functions Alerting the body to presence of insects on the skin Guarding the scalp against physical trauma, heat loss, and sunlight Distribution Entire surface except palms, soles, lips, nipples, and portions of external genitalia Hair Consists of dead keratinized cells Contains hard keratin; more durable than soft keratin of skin Hair pigments: melanins (yellow, rust brown, black) Gray/white hair: decreased melanin production, increased air bubbles in shaft Hair Follicle Hair follicle receptor (root hair plexus) Sensory nerve endings around each hair bulb Stimulated by bending a hair Arrector pili Smooth muscle attached to follicle Responsible for goose bumps Types of Hair Vellus pale, fine body hair of children and adult females Terminal coarse, long hair of eyebrows, scalp, axillary, and pubic regions (and face and neck of males) Hair Thinning and Baldness Alopecia hair thinning in both sexes after age 40 True (frank) baldness Genetically determined and sex-influenced condition Male pattern baldness is caused by follicular response to DHT
Structure of a Nail Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes 1.Protection three types of barriers Chemical Low ph secretions (acid mantle) and defensins retard bacterial activity Physical/mechanical barriers Keratin and glycolipids block most water and water- soluble substances Limited penetration of skin by lipid-soluble substances, plant oleoresins (e.g., poison ivy), organic solvents, salts of heavy metals, some drugs Biological barriers Dendritic cells, macrophages, and DNA 2.Body temperature regulation ~500 ml/day of routine insensible perspiration (at normal body temperature) At elevated temperature, dilation of dermal vessels and increased sweat gland activity (sensible perspirations) cool the body 3.Cutaneous sensations Temperature, touch, and pain 4.Metabolic functions Synthesis of vitamin D precursor and collagenase Chemical conversion of carcinogens and some hormones 5.Blood reservoir up to 5% of body s blood volume 6.Excretion nitrogenous wastes and salt in sweat